Add a disjoint sets (union-find) utility (#6797)

This will be used in an upcoming type optimization pass and may be
generally useful.

1 files changed, 87 insertions, 0 deletions

diff --git a/src/support/disjoint_sets.h b/src/support/disjoint_sets.h
new file mode 100644
index 000000000..198e5cb67
--- /dev/null
+++ b/src/support/disjoint_sets.h
@@ -0,0 +1,87 @@
+/*
+ * Copyright 2024 WebAssembly Community Group participants
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef wasm_support_disjoint_sets_h
+#define wasm_support_disjoint_sets_h
+
+#include <cassert>
+#include <cstddef>
+#include <vector>
+
+namespace wasm {
+
+// A disjoint set forest (a.k.a. union-find) implementation. See
+// https://en.wikipedia.org/wiki/Disjoint-set_data_structure.
+struct DisjointSets {
+  struct ElemInfo {
+    // The index of the parent element, or the index of the element itself if it
+    // has no parent.
+    size_t parent;
+    // An upper bound on the height of the tree rooted at this element.
+    size_t rank;
+  };
+  std::vector<ElemInfo> info;
+
+  // Add an element and return its index.
+  size_t addSet() {
+    size_t ret = info.size();
+    info.push_back({ret, 0});
+    return ret;
+  }
+
+  // Get the representative element of the set to which `elem` belongs.
+  size_t getRoot(size_t elem) {
+    assert(elem < info.size());
+    size_t root = elem;
+    // Follow parent pointers up to the root.
+    for (; info[root].parent != root; root = info[root].parent) {
+    }
+    // Compress the path to make subsequent getRoots of this set faster.
+    while (elem != root) {
+      size_t parent = info[elem].parent;
+      info[elem].parent = root;
+      elem = parent;
+    }
+    return root;
+  }
+
+  // Join the sets to which the elements belong and return the representative
+  // element of the union.
+  size_t getUnion(size_t elem1, size_t elem2) {
+    assert(elem1 < info.size() && elem2 < info.size());
+    size_t root1 = getRoot(elem1);
+    size_t root2 = getRoot(elem2);
+    if (root1 == root2) {
+      // Already in the same set.
+      return root1;
+    }
+    // Canonicalize so that root1 has the greater rank.
+    if (info[root1].rank < info[root2].rank) {
+      std::swap(root1, root2);
+    }
+    // Merge the trees, smaller into larger.
+    info[root2].parent = root1;
+    // If the ranks were equal, the new root has a larger rank.
+    if (info[root1].rank == info[root2].rank) {
+      ++info[root1].rank;
+    }
+    return root1;
+  }
+};
+
+} // namespace wasm
+
+#endif // wasm_support_disjoint_sets_h